The present invention relates to an electrochemical sensor element for determining the concentration of a gas component in a gas mixture, particularly for determining the oxygen concentration in exhaust gases of internal combustion engines.
An electrochemical sensor element of this type is described in, for example, German Patent No. 199 41 051. These types of sensor elements are used in gas measurement detectors known to those skilled in the art as broadband-lambda probes and are used for regulating the air/fuel ratio of combustion mixtures in motor vehicle engines. In these sensor elements, a concentration cell is combined with an electrochemical pump cell. The concentration cell has a measuring electrode located in a measuring gas chamber and a reference electrode located in a reference gas chamber and measures the oxygen partial pressure of the exhaust gas, which reaches the measuring gas chamber via a diffusion barrier, relative to the oxygen partial pressure of a reference gas in the reference gas chamber. The measuring gas chamber and the reference gas chamber are located in the same layer level of the sensor element.
The pump cell has a first pump electrode located in the measuring gas chamber and a second pump electrode located on a surface of the sensor element facing the exhaust gas and pumps oxygen ions from the exhaust gas into the measuring gas chamber or, conversely, from the measuring gas chamber into the exhaust gas. Through an external circuit element, the pump flow flowing through the pump cell is adjusted in such a way that a preset oxygen partial pressure is set in the measuring gas chamber. The oxygen partial pressure of the exhaust gas can be determined from the pump flow necessary for this purpose.
It is disadvantageous in the known sensor element that an undesired exchange of oxygen ions can occur between the electrodes of the pump cell and the electrodes of the concentration cell, which interferes with the intended function of the sensor. Thus, the oxygen partial pressure of the reference gas may be changed by the exchange of oxygen between the second pump electrode and the reference electrode. Since the concentration cell measures the ratio of the oxygen partial pressure in the measuring gas chamber and the oxygen partial pressure in the reference gas chamber, an incorrect oxygen partial pressure is set in the measuring gas chamber and therefore an incorrect value is determined for the oxygen partial pressure of the exhaust gas.
The electrochemical sensor element according to the present invention has the advantage relative to the related art that the exchange of ions of the gas component between the electrodes of the pump cell and the electrodes of the concentration cell is prevented by a barrier layer.
The barrier layer located between the reference electrode and the second pump electrode prevents the concentration of the gas component in the reference gas from being corrupted by the exchange of oxygen ions between the reference electrode and the pump electrode. The barrier layer is designed in such a way that the function of the concentration cell and the pump cell is not impaired. For this purpose, a cutout is provided in the region of the electrodes of the concentration cell and/or of the pump cell.